The present invention relates to a device for and a method of metering a predetermined amount of a particulate substance and to a metering apparatus comprising two or more of such metering devices. More specifically, the present invention is directed to the metering or dosing of granular or spheronized substances having poor flowability. The present invention is especially, but not exclusively, adapted to meter pressure-sensitive particulate substances, such as in the metering of a predetermined amount of a particulate pharmaceutical substance for use in a dry powder inhaler.
Powders having very small particle sizes (micronized powders) are commonly used in inhalation therapy. Such powders are often light and dusty, and therefore cause handling problems. Furthermore, such powders have very poor free-flowing properties which often make the handling and precise metering thereof problematic.
It is known to form such powders into larger particles (agglomerates) in order to enhance flowability. WO-A-95/09615 discloses one method of forming spheronized agglomerates which are capable of being broken down during inhalation to provide a fine powder. Such agglomerates consisting of more dense and compact particles present, however, another disadvantage in that the agglomerates are relatively pressure sensitive. This makes it difficult to meter the agglomerates without damaging the agglomerates and/or creating larger agglomerates, thereby reducing the flowability and hence the preconditions for exact metering.
Thus, there is a need for a technique which permits a more exact metering of fine particulate substances, in particular agglomerates. Exact metering is especially important in the case of handling and distributing powders containing medicaments where stipulated volume or weight tolerances must be complied with. One example is the filling of dry powder inhalers, such as inhalers of the kind disclosed in EP-B-0237507.
GB-A-2113182 discloses a metering device for metering granular materials. The disclosed metering device is, however, adapted to meter free-flowing granular materials, such as granular fertilisers, and not powders having very small particle sizes with poor flowability. The disclosed metering device includes a reservoir in the form of an elongate tube, and, whilst suited to the delivery of free-flowing granular materials to the metering mechanism, would be unable reliably to deliver powders having very small particle sizes to the metering mechanism, since the head of powder in the reservoir above the metering mechanism would result in physical bridges of the powder being formed in the reservoir, thereby preventing the free flow of powder to the metering mechanism.
It is thus a general aim of the present invention to provide a device for and a method of metering a particulate substance in a reliable and exact manner.
A further aim of the present invention is to provide a device for and a method of metering pressure-sensitive particulate substances, such as agglomerates, in a manner which avoids damaging the particulate substances.
Another aim of the present invention is to provide a device for and a method of metering particulate substances which can be performed at high speed.
Accordingly, the present invention provides a metering device for metering a predetermined amount of a particulate substance, comprising: a body member defining a metering chamber; an inflow path having an inlet and leading to the metering chamber, wherein the inflow path is unbroken, rotatable about an axis inclined relative to the horizontal and includes a first part which is co-axial with the rotational axis and a second part which is inclined relative to the rotational axis so as to be inclined downwards in a loading state and upwards in an emptying state, whereby the inflow path in use guides a gravity-induced flow of particulate substance into the metering chamber in the loading state and provides a gravitational inlet lock which prevents the flow of particulate substance into the metering chamber in the emptying state; an outflow path having an outlet and leading from the metering chamber, wherein the outflow path includes flow control means which prevents the flow of particulate substance out of the metering chamber in the loading state, but admits the flow of particulate substance out of the metering chamber in the emptying state; and a reservoir leading to the inlet of the inflow path, wherein the reservoir is rotatable and configured so as in use to rotate together with and provide particulate substance to the inflow path; characterized in that the reservoir is further configured such that on rotation a tumbling effect is achieved at the inlet of the inflow path for every volume of particulate substance containable therein.
The metering device is adapted to meter a predetermined amount of a particulate substance, either as a predetermined volume or a predetermined weight. In this application, it will be understood that the term state transition encompasses both a transition from the loading state to the emptying state and from the emptying state to the loading state.
A feature of the metering device is that the inflow path to the metering chamber is unbroken. In this application, it will be understood that the term unbroken is used to mean that no mechanical valve members or the like are disposed in the inflow path. Thus, damage to particulate substance present in the inflow path can be avoided, and the metering device is particularly suited to the metering of pressure-sensitive particulate substances such as powder agglomerates. The use of mechanical valve members downstream of the metering chamber is, however, not excluded so long as such valve members do not block or disrupt the outflow path. An example thereof will be described hereinbelow.
Another feature of the metering device is the provision of a gravitational inlet lock upstream of the metering chamber; that is, a locking function which results from the gravitational force acting on the particulate substance present in the inflow path. As a result, it is possible, even after filling of the metering chamber has been completed, to maintain unbroken contact between the particulate substance present in the inflow path and the metered amount of particulate substance present within the metering chamber. Thus, no mechanical valve member or the like is required at the inlet to the metering chamber to prevent the further inflow of particulate substance after filling has been completed.
In addition to the above-described gravitational inlet lock, the outflow path also includes flow control means which prevents the flow of particulate substance from the metering chamber in the loading state, but admits such outflow in the emptying state. As a consequence of the fact that the metering chamber, and the outflow path downstream of the metering chamber, will normally be empty during the filling step, the flow control means down stream of the metering chamber may, in contrast to the inlet lock, be in the form of a movable mechanical valve member or the like, the operation of which is controlled in a suitable manner in response to each state transition. However, in a preferred embodiment the metering device of the present invention includes a gravitational outlet lock downstream of the metering chamber. In order to provide a gravitational outlet lock, the state transition further involves a rotation of the outflow path so as to prevent the flow of particulate substance from the metering chamber in the loading state, but admit such outflow in the emptying state.
The flow paths preferably define a channel which extends through the body member. However, the flow paths may also be in the form of non-closed surfaces which support and guide the particulate substance, provided that such surfaces can be inclination-shifted for accomplishing the gravitational locking.
The present invention also provides a method of metering a predetermined amount of a particulate substance, comprising the steps of providing a metering device for metering a predetermined amount of a particulate substance, the metering device comprising a body member defining a metering chamber, an inflow path having an inlet and leading to the metering chamber, wherein the inflow path is unbroken, rotatable about an axis inclined relative to the horizontal and includes a first part which is co-axial with the rotational axis and a second part which is inclined relative to the rotational axis so as to be inclined downwards in a loading state and upwards in an emptying state, an outflow path having an outlet and leading from the metering chamber and a reservoir leading to the inlet of the inflow path, wherein the reservoir is rotatable about the rotational axis and configured so as in use to rotate together with and provide particulate substance to the inflow path; providing particulate substance into the reservoir; rotating the inflow path so as to incline the second part thereof downwards and guide a gravity-induced flow of the particulate substance therethrough so as to fill the metering chamber; preventing a gravity-induced flow of the particulate substance out of the metering chamber during the filling step; rotating the inflow path so as to incline the second part thereof upwards and thereby provide a gravitational inlet lock which prevents the further flow of particulate substance into the metering chamber; and emptying the metering chamber while particulate substance is prevented from flowing into the metering chamber; characterized in that the reservoir is further configured such that on rotation a tumbling effect is achieved at the inlet of the inflow path for every volume of particulate substance containable therein.
The present invention further provides a metering apparatus for metering a predetermined amount of a particulate substance, comprising at least one metering device comprising a body member defining a metering chamber, an inflow path having an inlet and leading to the metering chamber, wherein the inflow path is unbroken, rotatable about an axis inclined relative to the horizontal and includes a first part which is rotatable about the rotational axis and a second part which is inclined relative to the rotational axis so as to be inclined downwards in a loading state and upwards in an emptying state, whereby the inflow path in use guides a gravity-induced flow of particulate substance into the metering chamber in the loading state and provides a gravitational inlet lock which prevents the flow of particulate substance into the metering chamber in the emptying state, and an outflow path having an outlet and including flow control means which prevents the flow of particulate substance out of the metering chamber in the loading state, but admits the flow of particulate substance out of the metering chamber in the emptying state; characterized in that the metering apparatus further comprises a rotatable wheel having two or more metering devices mounted thereon in circumferentially spaced positions with a common rotational axis.
An advantage of disposing a plurality of metering devices on a rotatable wheel is that one metering device can be loaded at a loading station, while another, previously filled metering device can be emptied simultaneously at an emptying station. In a preferred embodiment the metering apparatus comprises at least two metering devices disposed diametrically opposite each other on the rotating wheel, such that when one of the at least two metering devices is in the loading state another is in the emptying state.
When using only one metering device, the outlet thereof can be held in a substantially fixed position in a horizontal plane during a state transition. As a result, the flow of particulate substance from the metering device is focused. In contrast, in the case of a rotating wheel provided with a plurality of metering devices which follow a circular path, the outlet of each metering device is not stationary during a state transition. In order to avoid the premature flow of particulate substance from the outlets during rotation from the loading state to the emptying state, each metering device preferably comprises an individually controllable valve member, which is movable between a closed position for closing the outlet of the metering device in the loading state and an open position for opening the outlet of the metering device in the emptying state. The operation of such valve members may be controlled by a cam-type arrangement which is configured to operate in response to the rotation of the rotatable wheel.